Fix onSquadSelectionPage dispatching for weapons

[Lemonymous]

Addresses #216

The event onSquadSelectionPage could be incorrectly dispatched when changing the weapon inventory page during a run. It will now only be dispatched while the squad selection window is visible.

The game remembers the squad selection pages for the squad selection page in the hangar and the statistics separately, so sdlext.squadSelectionPage and the local lastSquadPage has been been re-specified, so that both can be known at all times.

• sdlext.squadSelectionPage -> sdlext.squadSelectionPageInHangar | sdlext.squadSelectionPageInStatistics
• lastSquadPage -> lastSquadPageInHangar | lastSquadPageInStatistics

The existing code has also been refactored.

• Moved detection of squad selection page out of GetText, into its own function.
• Moved page update code out of GetText, and into its own onFrameDrawStart event.
• Removed custom windows.Hangar_Select.show function, and moved the logic to an onSquadSelectionWindowShown event.

Removed behavior that caused onSquadSelectionPageChanged to be dispatched, potentially several times over several frames, when opening the squad selection window. Replaced with logic that sets the correct page immediately as the window has been detected.

The 3rd argument dispatched with onSquadSelectionPageChanged has been removed. I could not find the documentation for this argument, but it appears to have been there to distinguish false from true page changes. As the new code does not dispatch false page changes, this is no longer needed.

Testing PR:

The following functions can be copy-pasted into the console to add debug logs, to test whether the events are dispatched correctly.

modApi.events.onSquadSelectionWindowShown:subscribe(function() 
	LOG("onSquadSelectionWindowShown on page", sdlext.getSquadSelectionPage()) 
end) 

modApi.events.onSquadSelectionPageChanged:subscribe(function(to, from) 
	LOG("onSquadSelectionPageChanged from", from, "to", to) 
end) 

modApi.events.onSquadSelectionWindowHidden:subscribe(function() 
	local page = sdlext.isHangar() and sdlext.squadSelectionPageInHangar or sdlext.squadSelectionPageInStatistics 
	LOG("onSquadSelectionWindowHidden on page", page) 
end)