Add metadata block to replays

The replay files are just streams all network communication so to
get any info out of them it is necessary to play back the stream
until the wanted information is reached.

This introduces a new metadata block placed at the end of the
replay files and logic to read the new block, or fall back to
playing back the stream for older files.

The replay browser is also updated to use the metadata information
instead of reading the replay stream directly.
Add filters to the replay browser dialog

This closes issue #2152. The filters added are:

* Game type (singleplayer / multiplayer)
* Date
* Duration
* Outcome
* Player name

Other changes:

* Added a 'CollapseHiddenChildren' option to the ScrollPanelWidget to
make hidden children take up no space.
* Removed the extension (.rep) from the replay filenames in the
replay browser.