Add a button to show/hide the messages widget #1001

[NIKEA-SOFT]

Task from #1001

[lmoureaux]

Simple change that improves the UI, I like it! Few comments:

• Don't worry about the wasm build, it's not your fault
• I can still move the messages using the small arrow at the top left. I think we should remove it.
• The expand/collapse button in the title bar duplicates the new show/hide functionality in the top bar. I would also remove it.
• The new icons won't follow the color scheme because they are in png format. With svg icons this code will automatically adjust the color (it's quite dumb: it replaces #000 everywhere in the file with another color). This way the icons can match the theme.
• Speaking of icons, we should add .license files for the new files (look at data/themes/gui-qt/icons for examples)
• Could we have a red dot on the notify icon? I think it would make it more distinct