Cache icons in TorrentContentModel

This commit caches icons in TorrentContentModel in the same way they are
cached in TorrentModel.

This commit should improve performance when user navigating through
torrent list using up/down keys. A scrolling through all the list
(276 torrents) took:

    Total wall time:                            18.813s
    Total CPU time:                              3.210s
    IconProvider::generateDifferentSizes():      0.170s

IconProvider::generateDifferentSizes is 5th most hottest function on
this use case.
Cache QRegExp in misc::parseHtmlLinks()

This commit should improve performance when user navigating through
torrent list using up/down keys. A scrolling through all the list
(276 torrents) took:

    Total wall time:                            18.813s
    Total CPU time:                              3.210s
    misc::parseHtmlLinks():                      0.096s

misc::parseHtmlLinks() is 8th most hottest function on
this use case.
Merge pull request #2144 from sorokin/up-down-scrolling

Speed up scrolling with up/down keys