Improve version.pri.

Now you can set a version number only in one place.
Change indentation to 4 spaces. Issue #2192.
WebUI: Assign a version to the Web API and allow to get it

New GET methods:
 * version/api: current API version
 * version/api_min: min API version supported

The API version is an integer and it's returned as plain text.

Whenever the API is changed, the api version is increased.

Whenever the API is changed in a way that is not backward compatible,
api_min is increased.
WebUI: Allow to retrieve qBittorrent version

New GET method:
 * version/qbittorrent: qBittorrent version

Closes #1872.
LINUX: Fix build with libtorrent 1.0.x. Closes #1878.
Make configure search for sed.
Fix QtBus detection with configure.
Try to fix FreeBSD detection with configure.
Make configure detect qmake with different names. Closes #1924.
Merge pull request #2189 from glassez/version

Improve version.pri to set a version number only in one place.
Use VERSION_BUILD where possible. Revert a change from previous commit.
Merge pull request #2125 from buinsky/master

WEBUI: Some fixes
Merge pull request #2153 from pmzqla/webui-versioninfo

WebUI: Allow to retrieve web API version and qBT version