Make tabline scrollable

[evgenykochetkov]

Closes #1087
Closes #414

[nkrkv]

Works like a charm! The code is fine.

A few things I spotted that annoy a bit:

1. If I open a new tab when the tabline is overflown already, I don’t see the newly activated tab. In particular, this would cause confusion when the debugger tab opens.
2. If I click a tab which is half-hidden/half-visible, it “blinks.” It shows completely when the button is held down and disappears when the button is released. Looks like it is caused by drag-n-drop
3. The < > arrows are misaligned vertically with tab contents

(1) and (2) can be solved by automatically scrolling to a tab being clicked or activated. By scrolling to I mean ensuring it is completely fit in the visible tabline area.

(3) Looks like a problem of tabs, not the buttons. Should we push tab contents up a couple of pixels?

[brusherru]

🐛 Bugs detected:

1. I can't scroll any sidebar pane if I opened only one. (Show only ProjectBrowser and try to scroll it)
2. The previous bug moves Deployment pane out of view area
3. Even If I open Inspector and Help panes in the one sidebar, it moves Deployment pane and I can't see an entire Help pane content.

Here's a GIF with all these bugs:

[brusherru]

🐛 One more bug:
When I open a new tab and tab panel scrolled to the active tab I see a native scrollbar 🙈

And it reproduces with scrolling...

[brusherru]

🐛
Also, padding over the tabs is disappearing right after IDE loads :)

[brusherru]

Possibly, bug 🤷‍♂️

1. Open a lot of tabs
2. Scroll tabs to the left
3. Scroll patch board
   Tab pane will scroll to the active one. Is it okay? :)

[brusherru]

About design 🤓

1. It's a little annoying when tabs scroll so fast (jumps) to the active one. Especially, if I click on the first item in the view area, which did not fit the container entirely.
   Maybe better to make a smooth scrolling? :)
2. Button has a background color and sharp edges. So tabs that do not fit the container look ugly. What about adding a gradient or a slightly light stripe, or something else? 🤔

[nkrkv]

LGTM 🎈

[brusherru]

👍