CSS fixes and improvements

[piotrpalek]

The main issue I was trying to fix is that the "Run Tests" and "Live Reload" checkboxes cover each other when "Live Reload" is deselected:
Before:

After:

Other problems this will fix (discovered while fixing the first issue):

• the header elements are consistently positioned (especially vertically) across all panes (esp. visible for eg. the on "Output" string):
  Before:
  
  After:
  
• when the tree is hidden and there are 0 editor panes the page looked broken:
  Before:
  
  After:
  
• I've changed how the file tree looks and behaves, it is no longer resizable and is much smaller which leaves more space for the important stuff (it still expands when the tree shifts to the right due to many items):
  Before:
  
  After:
  

Mobile view (only last screen has real changes so I provide a "before" screenshot there):

Fixed output pane:
Before:

After:

Edit: I think I'd add a little more spacing to the file tree (left/right) looks a bit too cramped together imo

[Gaurav0]

1. Please don't make the file tree unresizable. Making it smaller by default would be fine.
2. Please post mobile screenshots after changes.

[piotrpalek]

@Gaurav0 Regarding the first question, what's the utility of being able to resize the file tree?

edit: I know text editors generally have that ability, just trying to think of a valid reason why :) And if we should provide this functionality for the file pane as well.

edit2: On the other hand I don't really feel strongly about this, if we ever want to disable the file pane dragging we can do that easily. I've restored it for now in the latest push.

[piotrpalek]

@Gaurav0 I've updated the description with mobile screenhots. The changes are also visible here

edit: pushed a few fixes for the filepane, main one was being able to drag the pane above the tree:

[piotrpalek]

@Gaurav0 noticed one more issue, the filepane behaves a bit weirdly when trying to shrink it below the minimum width (instead of nothing happening, the panel next to the filepane starts to grow). Let me fix that before merging.

[piotrpalek]

@Gaurav0 I've fixed the tree issue and some additional ones:
edit: ready to merge from my side (changes reflected here as well)

• added a scrollbar when the filepanes contents are too big
  Before:
  
  After:
  
• the top toolbar was broken in mobile view when the text was too long (for eg. when displaying the revision)
  Before:
  
  After:
  

[Gaurav0]

Looks Awesome. Thanks!