README: convert all links to markdown format.
README: change Qt website to qt-project.org.
Fix: PoweroffDialog not centered if message is too long.
PoweroffDialog: put program icon beside the message.
Update the acknowledgement of icons in README.
README: wrap lines at 80 chars.
Update manual page.
Makefile: add "all" as the default build target.
Makefile: set PREFIX only if it doesn't exist.

Use ?= to assign default prefix to PREFIX.
Add application icon for OS/2 Warp.
Remove "AVI Auto" preset and use "MS Compatible AVI" as the default.
Recursively add directories when adding files.
AddTaskWizard: reuse Extension object.
Extensions: add contains() method.
Ignore file extensions not known as media files in recursion.

When recursively adding files, exclude every file whose extension
is not in the multimedia extension list (union of video and audio).
AddTaskWizard: always show the "Files" page.
ConvertList: call addTasks() only if wizard returns Accepted.
AddTaskWizard: remove dependency to QUrl.
Filter out unknown extensions in drag-and-dropped files.
Drag and drop: don't show AddTaskWizard if no file is accepted.