Minor improvements to the pane's tab style

[javier-tarazaga]

Description of the Change

Added the following changes:

• Make the entire tab space a little bit smaller to gain a few more pixels of real state for the editor.
• Added a selected tab underline to clearly see which is the selected tab.
• Minor color and padding fixes for the x button in the tabs.

[filippsen]

• "Make the entire tab space a little bit smaller ...": thinner tab and style changes, OK
• "Added a selected tab underline...:": was this the one removed on the very last commit (7eb3df7) ? If so, OK.
• "Minor color and padding fixes for the x button...": subtle effect on mouse over and style changes, OK.