EXPERIMENT: display the Find/Replace overlay as partstack

[Wittmaxi]

Experiment with display the Find/Replace Overlay as a Part in a Partstack instead of an overlay shell.

Caveats:

• the part has the annoying tendency to stick to the side instead of allowing to be "positionned to the top all the time"
• keypresses are still interpreted as keypresses targeted towards the Editor behind (ie. pressing Ctrl+Backspace delets a full word in the editor not in the overlay)

First time opening the overlay (something is wrong...)

Second time pressing Cltr+F

Adapted from #2093

I do not believe that this solution can bring improvement to #2099

[github-actions]

Test Results

0 files   -  1 815  0 suites   - 1 815   0s ⏱️ - 1h 37m 11s
0 tests  -  7 696  0 ✅  -  7 468  0 💤  - 228  0 ❌ ±0 
0 runs   - 24 249  0 ✅  - 23 500  0 💤  - 749  0 ❌ ±0 

Results for commit 4f8aac4. ± Comparison against base commit b96413b.