envvars-dialog: Allow to paste var=value and fix validation

[jntesteves]

• Rework the envvars dialog to allow copy/pasting complete variable assignments instead of having to input first the name and later the value.
• Fix the input validation which had some inverted logic and make it behave as expected.
• Make the panel leaner by removing unnecessary headers and periods at the end of labels.
• Port to AdwDialog

[TheEvilSkeleton]

Some things that should be fixed:

• Inserting a character and removing it will keep showing the check button:
  
  Instead, the check button shouldn't appear.
• The moment you type something, the row turns red, and the check appears:
  
  Instead, the row should keep the neutral color without displaying the check. The error should only appear when the user explicitly adds the equal sign (=) and removes it.
• If you remove the last defined environment variable, the "No environment variables defined" label doesn't appear, when it should.
• It's pretty easy to accidentally close the dialog without saving your changes, so there could be an AlertDialog to confirm the user intends to discard.
• I think the dialog is too big for what it is, perhaps you could adjust the height to content-height: 550; and content-width: 500;?

[jntesteves]

* Inserting a character and removing it will keep showing the check button:
  Instead, the check button shouldn't appear.

* The moment you type something, the row turns red, and the check appears:
  Instead, the row should keep the neutral color without displaying the check. The error should only appear when the user explicitly adds the equal sign (`=`) and removes it.

Done. A tiny bit hackey because of the way the apply button works. The widget doesn't have a method to just hide it without outright disabling the feature. So I disable+enable it just to reset its state when invalid. Works beautifully :chefs_kiss:

* If you remove the last defined environment variable, the "No environment variables defined" label doesn't appear, when it should.

Done.

* It's pretty easy to accidentally close the dialog without saving your changes, so there could be an [AlertDialog](https://gnome.pages.gitlab.gnome.org/libadwaita/doc/main/class.AlertDialog.html) to confirm the user intends to discard.

This is not a regression but an issue this dialog already had, so I think it can be deferred to another PR as to not block and delay the improvements in this one.

* I think the dialog is too big for what it is, perhaps you could adjust the height to `content-height: 550;` and `content-width: 500;`?

Now that the variable names are on the same line, some values on longer variables don't fit the screen. So I increased the width from the previous 500 to 600 to give more space for the names. That width is still a bit smaller than the width of the rows on any settings screen, which I think is a fair width for properties otherwise it wouldn't be the default on all the Adw properties widgets. The dialog is responsive so this is the max width, it scales down when space is limited.

As for the height, we have an issue that AdwDialog is not resizeable, so dialogs that will potentially contain long lists have to be tall. Again, they are responsive and scale down on small screens.

[TheEvilSkeleton]

Good point on every disagreement. I'll take a look at the code

[TheEvilSkeleton]

Everything looks good to me. The only thing that needs to be done is updating the POTFILES file. After that, you're free to merge.

[jntesteves]

Everything looks good to me. The only thing that needs to be done is updating the POTFILES file. After that, you're free to merge.

Is this what you mean? I've regenerated the translation files. It seems that hasn't been done in a while, it made hundreds of changes coming from many changed files.

[TheEvilSkeleton]

My apologies, I thought you changed the file names but my eyes deceived me. Feel free to revert it 😅

[TheEvilSkeleton]

Everything looks good to me. Merging :)