Enable high-frequency tooltip updates; Bump GTK+ version

List of changes:

• Enable high-frequency tooltip updates (#11 (closed)):

  • This patch enables the tooltip to be shown when the plugin update timeout is smaller than the GTK+ tooltip popup delay (500 milliseconds)
  • The patched code can be expected to be faster, because:
    • It isn't re-creating the tooltip widget on every plugin update
    • It isn't updating the tooltip widget text if no tooltip widget has been created yet
  • The old code was using gtk-tooltip-timeout which is deprecated since GTK+ 3.10

• Code cleanup: Remove "!" debug messages

• Bump required GTK+ version to 3.16

  • GTK+ 3.16.0 was released in year 2015

Closes: #11 (closed)

Mitigates: #1

Mitigates: #5