4th cleanup

Highlights
- Filter more unwanted windows (especially a persistent explorer.exe window)
- Display pretty names for more apps (ex. Github Desktop)
- Fix autorun dialog spam on startup
- Much prettier code

Play-by-play
- refactor: introduce style_t
- refactor: rename windows_t field count -> num_windows
- refactor: add windows_t field 'num_apps' (see below)
- refactor: remove selection.active, use selection.window instead
- cleanup:  small formatting fixes and comments
- refactor: simplify title() after realizing GetWindowTextW() returns length
- feature:  appname(): fallback to Product Name if File Description is missing
- cleanup:  remove a bunch of commented-out code in filter()
            - THIS COMMIT MAY CONTAIN USEFUL WINDOW FILTERING TECHNIQUES WORTH
               RECOVERING AT SOME POINT
- refactor: simplify several functions:
            - remove numapps(), use new windows.num_apps instead, set by link()
            - remove width(), move functionality to size_gui()
            - remove link(), move functionality to add()
            - remove wipe(), move functionality to rebuild()
- refactor: other changes to functions:
            - size_gui() takes a style_t parameter
            - possibly a slight perf increase in add() (not measured)
            - reset windows.num_apps in rebuild()
- feature:  filter more unwanted windows
            - especially a persistent hidden explorer.exe window is hidden now
- debug:    print class name in print_window()
- debug:    option to print extended window details when using print_all()
- refactor: return NULL from find_next if window parameter is NULL
- cleanup:  remove commented-out code in select_next()
            - this code was left here as an illustration of how a prior refactor
               would look at this site. The prior refactor also moved the
               location of this logic to LLKeyboardProc; the refactor was never
               active at this location
- refactor: remove defensive check in select_done()
- note:     the redraw() function does not use style_t yet
- refactor: only resize gui on init
- bugfix:   improve NULL robustness when window-switching by checking
             selection.window
- bugfix:   fix autorun dialog spam on startup

Postscript
- commit message for aeee53b ("Second cleanup") missed:
   "refactor: remove should_switch & open_gui args from select_next()