Add 24-bit colour control sequence support

For detailed description of those control sequences, see here:
https://github.com/robertknight/konsole/blob/master/user-doc/README.moreColors
Fix true color attr losing while scrolling bug

Mainly add true color attribute handling code into line compress/uncompress code
Do not put back last window when switch to a background window.
Add .gitignore.
Merge branch 'master' of https://github.com/6bb79df9/mintty into pulled_changes
Merge branch 'master' of https://github.com/zcbenz/mimtty into pulled_changes
Merge branch 'pulled_changes'