ui: don't make blocking RPCs in the window event thread

We do this in a couple few places:
1. to read the sheet tag lines, for example, when finding the +output sheet. However, we will be reading more tag lines for other purposes soon.

We should fix this by using a view to track the tag line across changes.
2. to read the column number when moving the cursor up and down.
3. when reading the word under the mouse pointer when 2-click-executing a command

We should fix these two by using a separate go routine for textBox event handling. There is already a goroutine reading the notify channel of the textbox's view. Why not do it in there.

Start by tracing the window event thread and the textbox event handling, especially the existing goroutine that reads the notify channel of the textbox's view. Inspect the paths that read sheet tag lines, column numbers, and the word under the mouse pointer. Done means these event-handling paths no longer make blocking RPCs in the window event thread.

Written by the indexing model from the issue text.