Second cleanup

Highlights
- Fix drawing bug (white rectangle in switcher GUI)
- Cancel window switching by clicking outside switcher
- Confirmation dialog when quitting cmdtab
- Prettier code

Play-by-play
- refactor: use typedef for vkCode
- cleanup: pretty comment headers
- cleanup: remove/reword/reflow more comments
- cleanup: various minor style changes
- refactor: rename config_t field show_delay -> gui_delay
- refactor: declare winapi callbacks
- refactor: isolate alt hack
- cleanup: add & use 'countof' macro
- cleanup: rename 'handle' variable -> 'process'
- refactor: split width() into numapps() & width()
- refactor: pass 'selection' as arg to gdi_redraw() (avoid global access)
- refactor: merge gdi_init() & gdi_redraw() into redraw()
- refactor: remove defensive checks in show() & toggle() regarding foreground
            window
- refactor: simplify show()
- refactor: simplify timer handling in toggle()
- refactor: split toggle() into size_gui(), show_gui() & hide_gui()
- refactor: remove some (hopefully redundant) checks in filter()
- refactor: encapsulate window enumeration in new function rebuild()
            - move filter() (and debug printing) from add() to new function
               EnumProc()
            - change add() return type from bool -> void
            - move EnumWindows() call from link() to EnumProc()
- refactor: move windows->count reset from link() to wipe()
- refactor: move selection init code from cycle() to LowLevelKeyboardProc()
- bugfix: fix drawing bug caused by timer delay in gui-drawing logic in cycle()
- refactor: move gui-showing code from cycle() to LowLevelKeyboardProc()
- refactor: remove hide_gui arg from done() (always hide gui)
- refactor: change done() return type from bool -> void
- refactor: simplify set_key()
- refactor: disable CallNextHookEx() calls in LowLevelKeyboardProc (REGRESSION?)
- refactor: move defaults() initialization from WndProc() to wWinMain()
- feature: cancel window switching by clicking outside switcher
- feature: confirm when quitting cmdtab
- refactor: simplify wWinMain
            - inline & remove is_single_instance()
            - refactor & rename prompt_autorun() -> autorun()
            - greatly simplify window creation
- refactor: remove TranslateMessage() from message loop
- refactor: summary of function renames
            - show  -> show_hwnd
            - toggle_now -> TimerProc
            - dump  -> print_all
            - find  -> find_hwnd
            - first -> find_first
            - last  -> find_last
            - next  -> find_next
            - cycle -> select_next
            - done  -> select_done
            - LowLevelKeyboardProc -> LLKeyboardProc
            - prompt_autorun -> autorun
- header: don't include SDKDDKVer.h (REGRESSION?)
- header: add 'dbg_print' macro (not used yet)