Fix: The window title wasn't displayed when "Display full path" option was used.

This option was broken after bd905fce40009a950c754486133ab89da2c9919d.

This commit fixes ticket #3394.
Fix: the window title was wrong in capture mode when "Display full path / file name" was used.
Cosmetics: Don't use "m_" prefix for function variables.
Use pipe:stdin instead of pipe:0 to get clearer OSD message and window title.
PlayerSeekbar: utilize last WM_MOUSEMOVE point

In mouse capture mode, and seek only if new x coordinate differs.
It fixes madVR with delayed seek option enabled seeking to the same
point repeatedly.
PlayerSeekbar: code refactoring follow-up

A few minor things I missed in 444d7e9
Modify goto subtitle behavior

Jump only if there's a subtitle present. Previously mpc-hc performed 5s
jump if there was no subtitle, even beyond the end of stream.
New "Jump to Beginning" command
Fix: the window title was wrong in DVD mode when "Display full path" was used.
WebServer: Add margin-top to seekbar.