[Core] Changed ENABLE_CLANG_QUIRKS semantics.

Enable clang quirks by default for Clang compiler, also allow one to
enable it for arbitrary compiler.
[Util] Fixed ResourceLoader with caching on large pixmaps.

QCache::insert() can delete the passed object immediately, so we need
to make a copy.

clang-analyzer helper me to spot this.
[Lads] Check if LC menu is actually found.

clang-analyzer helper me to spot this.
[Blogique Metida] Properly handle unknown poll types.

clang-analyzer helper me to spot this.
[BitTorrent] Eliminated some dead code.

clang-analyzer helper me to spot this.
[Monocle Dik] Removed double assignment.

clang-analyzer helper me to spot this.
[MT] Fixed possible double-free.

clang-analyzer helper me to spot this.
[Monocle] Fixed too long object lifetime.

clang-analyzer helper me to spot this.
[Azoth Murm] Support removing accounts.
[XSD] Schedule cleanup instead of immediate cleanup.

Speeds things up threefold on accounts removal in Murm.