Changed numerical values from scale to spinbutton to fix issue #67

[benoitv-code]

Fixes #67:

In newer Cinnamon updates, the scale component is not displaying numbers anymore, which makes it hard to pick the desired number of minutes we want for a Pomodoro session for example.

This change replaces all "scale" components by "spinbutton" components for selecting specific numbers (minutes or number of pomodori in a session).

Tested on Cinnamon 3.2.7, not tested on previous versions (if someone can test for me it would be great).

[benoitv-code]

Note: Numbers on sliders are back on Cinnamon 3.4.4 (Linux Mint 18.2 released on 2017-07-02), but I feel spinbuttons make more sense to select minutes / numbers of pomodori, so I would still recommend going forward with this pull request.

[gfreeau]

@kunnix The problem with using spin buttons is it doesn't work in mint 17 which is still a LTS release. (cinnamon 2.8).

[benoitv-code]

Ok, keeping this commit here if things are diverging too much and you want to create 2 branches at some point (mint17 / master).