Add close sketch option

[sevsev9]

Is your feature request related to a problem? Please describe.
There is no proper way to close a sketch or any other window really in the IDE. So it just keeps filling up the window bar until you close the IDE.

Describe the solution you'd like
A small "x" on every tab open and the CTRL+W shortcut to close focused tabs fast.

Describe alternatives you've considered
Maby also add to context menu of the tabs' header.

Edit: There seems to be an "x" on the top right but it is invisible plus it only exists on the preferences tab (at least in v0.0.2)

[svenefftinge]

It is intentionally omitted in the default classic mode, as it is meant to be used on a single sketch that has all its files open. You can switch to advanced mode, where the close option is present (the cross in the tab, see screenshot).

[sevsev9]

In this case I would recommend though that you would automatically open a sketch(project) in a new window so that it is not as confusing.