Pathbar: Allways reserve space for two slider.
This avoids the situation that the pathbar may disapar if the window
was resized without slider buttons and slider buttons are required later.
Pathbar: replaced some 4 and 8 space indents by tabs
Pathbar: simplified required space calculation
Pathbar: renamed first_botton to pathbar_root_button

Name was ambiguous because it was the first displayed button in LTR order,
but the last displayed button in the button list.
Pathbar: fixed free space calculation when scrolling
Pathbar: use g_source_remove() for the initial scroll timer
Pathbar: Fixed drawing in comments