Click and double-click hotkeys

[elexis]

The current click and double click implementation is messy.

1) The usual GUI objects are handled in CGUI.cpp and it implements a custom click-delay measurement to decide if there was a doubleclick, instead of parsing the clicks count of the ​SDL_MouseButtonEvent.

2) Double-clicking in running games is again implemented with a custom timer in input.js, instead of receiving a more low-level mousebuttondouble event constructed in ScriptInterface::ToJSVal<SDL_Event_> of GuiScriptConversions.cpp.

3) Furthermore the meaning of click and doubleclicks is hardcoded in input.js (depending on the inputState). It might be preferable if players could chose on their own what happens when clicking / doubleclicking and also assign alternative hotkeys to trigger the actions that are currently only possible with clicking. The actions defined in input.js should be moved to functions triggered from gui/session/hotkeys/ and subscribe to new hotkeys events defined in default.cfg that are declared in KeyName.cpp and constructed in HotkeyInputHandler of Hotkey.cpp.

[elexis]

In r19444:

Replace input.js doubleclick hack and ease doubleclicking on moving units.
Differential Revision: ​https://code.wildfiregames.com/D326
Patch By: causative
causative Refs #4414

[elexis]

In 19583:

Fix doubleclicking if the unit died between the clicks following rP19444, refs #4414, ​https://code.wildfiregames.com/D326

Differential Revision: ​https://code.wildfiregames.com/D469
Patch By: causative

[elexis]

In 19786:

Prevent unintentional selection of all units following rP19444 / D326 when doubleclicking while not in selection input mode.

Differential Revision: ​https://code.wildfiregames.com/D634
Fixes #4631.
Refs #4414.
Patch By: causative

[elexis]

Also requested that we can customize the duration between clicks and pixel distance: #4741.