task-list-view: Done button has maximum width
This patch adds a maximum width to the Done button in the task list view.
A subclass of the GtkButton is created. This class is named GtdDoneButton. In this class the get_preferred_width() method is overrided by gtd_row_get_preferred_width_with_max() from gtd-commons.
The type of the Done button is changed in list-view.ui from GtkButton to GtdDoneButton and also the halign(center) property is added.