refactor: extract GUI-specific logic to GUI folders

[tlambert03]

this is a bit of a reversal of #75 ... but probably much better than the state of things before #75.

basically: trying to get all the gui-specific stuff back into their own folders. This creates a base NDVapp class that GUI frameworks have to implement to be supported. (its the same as the current GUIProvider object).

it does move a little bit of logic for each framework back into _app.py, mostly the "is_running" logic. but by having just that logic there, it allows all the rest to go back to the proper folder

this also fixes a bug wherein if you explicitly use NDV_GUI_FRONTEND=qt inside a jupyter notebook. it still doesn't let you use a Qt widget.

[codecov]

Codecov Report

Attention: Patch coverage is 67.02509% with 92 lines in your changes missing coverage. Please review.

Project coverage is 72.43%. Comparing base (2fdb603) to head (3e07907).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/ndv/views/bases/_app.py	58.66%	31 Missing ⚠️
src/ndv/views/_qt/_app.py	75.36%	17 Missing ⚠️
src/ndv/views/_wx/_app.py	67.30%	17 Missing ⚠️
src/ndv/views/_jupyter/_app.py	60.00%	16 Missing ⚠️
src/ndv/views/_app.py	73.80%	11 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #112      +/-   ##
==========================================
+ Coverage   72.29%   72.43%   +0.14%     
==========================================
  Files          44       48       +4     
  Lines        4595     4622      +27     
==========================================
+ Hits         3322     3348      +26     
- Misses       1273     1274       +1     

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