Add button to switch the timeline visibility

[dacap]

From a long time ago (~2013) I wanted a button to show the timeline (right now it's only accessible via View > Timeline, Tab, or automatically displayed when a new layer or frame is added).

Found the original mockup ( also in #1731 (comment) ):

We have a filmstrip icon in the theme too:

[Akylzhan]

I would like to work on this but I don't see filmstrip icon in the theme. Also seems like some big chunk of code will be duplicated (e.g drawing icon) should I move them to separate function or lambda?

[dacap]

Hi @Akylzhan, it would be great if you can take this one.

About the theme, check the icon_frame slice, I think we can use that one:

About the duplicated code, probably a private member function in ToolBar would do the job (we prefer member functions instead of lambdas if possible).

[Akylzhan]

Should I reuse icon_frame and invert the color or draw a similar one near preview icon?