Improve wall - impassable terrain construction

[scythetwirler]

When beginning wall placement it would be good to allow the player to begin placement in a "red zone", that is, allow the placement to begin in a location that a wall could not actually be placed, and then as the player moves the mouse to the end point the wall placement should begin at the first point between the potential end point and the beginning point placed in the "red zone".

As it is now, the wall placement requires too much time to begin, where the player is forced to carefully line up the beginning of their wall with the end of a "red zone" so as to ensure there is no leak through which a unit could pass through.

If there are two "red zones" (such as two non-passable cliffs) that a player would like to wall off, it would be nice to quickly click in the middle of one of these "red zones" and then click in the ending "red zone" and have the engine figure out where the wall is actually going to begin in the same way that it figures out where the wall should actually end.

Source: ​https://wildfiregames.com/forum/index.php?/topic/21297-fix-annoying-wall-placement-behavior/

[bercik9991]

I read the article mentioned as source of this idea and I agree with the comment (​https://wildfiregames.com/forum/index.php?/topic/21297-fix-annoying-wall-placement-behavior/&do=findComment&comment=321248).

It would be convenient to have trees and some other objects disappear when placing walls (maybe other buildings too).

For now, player must either cut trees to create space for them or just aim with mouse precisely looking for perfect place and path (which is a tedious and time-comsuming task).

P.S. If this comment does not follow the rules, let me know, I'm new to all this stuff but I'm just happy I could contribute so 0 A.D. becomes best RTS ever (in fact, it already is better in many ways and concepts than other games)

[elexis]

Refs #3789, perhaps it can be solved in the same go.

[Imarok]

Move tickets to In-game UI as UI & Simulation got some sub components.