fix borderless drag issues and add debug overlay to better test issues about draging area

[Simon-Laux]

add `exp.showDragAreas()` development api to make drag areas visible.

red is drag, green is no-drag.
How to use: go into electron dev tools, execute exp.showDragAreas() to toggle the overlay which makes drag areas visible.
This is a useful tool to make the issues with it more tangible.

Fix some of the bugs:

• make dialogs no-drag
• add input and button elements in case we missed cases, they should always be clickable
• make background image on "no account selected"- and welcome screen a drag area
• don't make dialog headers drag areas
• remove unused z-index css code from context menu
• make context menu not dragable

todo? or make new issue:

• Fullscreen media view -> where can I drag that?

closes #4018
closes #3926

[WofWca]

I have checked that the added DragRegionOverlay is not relevant in production. But it would also be nice to not include its code in the production build.

As to essence of this MR: I haven't checked it thoroughly, because I'm not on Mac.
But in general not so many lines have been changed.

[r10s]

this is an impressive layout, thanks for moving forward on that issue; in general, beside fullscreen, that seems to work on quick testing :)

Fullscreen media view -> where can I drag that?

i would expect that to work in the upper area of the screen, where normally the title bar is - similar to welcome screen

i cannot say much to the code, however :)

[nicodh]

Fullscreen media issue can be solved in a later fix imho

[r10s]

Fullscreen media issue can be solved in a later fix imho

agree, but we should file an issue for that then

[Simon-Laux]

So todo to merge this pr:

• add changelog entry
• file issue for the remaining issue with the full screen media view.