Allow for custom tab bar ids

[jtpio]

Problem

For now the set of tab bar ids is defined here:

galata/packages/galata/src/galata.ts

Line 100 in d717e6a

type SidebarTabId = 'filebrowser' | 'jp-running-sessions' | 'tab-manager' | 'jp-property-inspector' | 'table-of-contents' | 'extensionmanager.main-view' | 'jp-debugger-sidebar';

It would be great if this could be made configurable, or be able to pass any id.

For example the @jupyterlab/git extension adds a panel to the left area, and this extension could use galata for end to end testing.

Proposed Solution

Allow arbitrary ids (as strings)?

Additional context

Related to #45

[jtpio]

There might actually be other parts that could be made more customizable, to make it easier to use galata for third-party extension testing.