Replace ClickOverlay enterMonitor reactive actor approach with much simpler implementation

[jtaala]

@Lythenas, @Thesola10, I've been thinking about this one for a while:

PaperWM uses a very curious approach to implement a "one space per monitor" paradigm (see my response to a question of how PaperWM does this).

Unfortunately this approach is quite complex and exhibits many side-effects that have required many workarounds (which often causes further issues).

This PR replaces this approach with a simpler PointerWatcher implementation (a Gnome supported implementation to monitor mouse pointer movements). Moving to this approach provides the following advantages:

• switch to a Gnome supported feature to detect when mouse pointer moves to another monitor (instead of an unsupported, custom, error-prone implementation);
• significantly simplifies the codebase for PaperWM's multimonitor support;
• replaces around ~164 lines of (now) unneeded code with ~31 lines;
• removes a complex part of multimonitor support and would allow us to add simple behaviours (based on user preference) like current windows lost focus when mouse move across monitor #389;

Note: we actually already use the PointerWatcher approach for multi-monitor drag/drop support - so this PR is essentially removing the other approach and adding a "activate space" method to the PointerWatcher.

@Lythenas and @Thesola10 - could you give this PR branch a test? I'll set it as a draft atm given it's quite a large change and I'm sure there might be some side-effects from this.

@Thesola10 - I note that this may impact some of your touch signals, although, we no longer have the ClickOverlay element blocking actions so it may turn out better (just a hypothesis).

[Thesola10]

okay, the touch workspace switching is working a lot better. No issues to report.

Does not seem to impact #735 whatsoever.

[Lythenas]

The code looks good. To be honest it seems so simple compared to the click overlay...

I will use this for the next couple of days to see if I notice any issues.

[Lythenas]

Seems to work fine. I didn't notice any problems.

[Thesola10]

I may have failed to properly test this PR. As it turns out, this seems to introduce a regression of #655.

[jtaala]

I may have failed to properly test this PR. As it turns out, this seems to introduce a regression of #655.

Thanks @Thesola10,

My guess is that we may need to add a touch-event listener on space.background, similar to what was on the enterMonitor element.

I've added this on PR #755 (see commit 21efa47).

Note this may not do anything (and I don't have a touchscreen to test this on). Can you test 21efa47 and see if it responds to touch events on another monitor (it should activate that space on touch with this change). It may help in finding out more info here (or do nothing).