Configurable message actions

[cketti]

Different people use email in very different ways. Some actions on messages (delete, archive, mark as unread, move, reply, …) are very important to one group of users, but are never used by others. K-9 Mail currently offers limited support to alter the list of actions that are displayed in the toolbar when viewing a message. This can be configured under
Settings > General settings > Display > Visible message actions.

The existing feature isn't very flexible and works in ways that is surprising to many users. I suggest to change it so actions can be freely reordered or completely removed from the toolbar. The user interface for configuring message actions could be a list consisting of three sections. The first one contains the items that are displayed as icons in the toolbar, limited to a sensible number (maybe 4 or 5). The second section contains items that are displayed in the overflow menu (three dots). The third section contains additional actions that the user doesn't want to display at all.
Items in the list can be reordered using drag and drop, which also allows moving an item from one section to another.

The fact that we have sub-menus ("Send…" and "Refile…") makes this a bit more complicated. For a first implementation I suggest we keep the items of the sub-menus fixed and only allow the sub-menus itself to be reordered. The contents of the sub-menus should also exist as single items that can be placed individually. This may lead to an action being available twice in the toolbar, e.g. "Move" as icon and then as part of the "Refile…" sub-menu. But the same is true with the current implementation.

Another complication is the fact that we support a split-screen view where the message list and the message contents are displayed next to each other. In this view we "merge" the toolbar actions. Suggestions on how to best handle this situation are welcome.

Related: #3530

[llucax]

#5448 seems related and brings a bit more urgency either to fix that issue or this one, as currently there seems to be no way to configure the actions at all.

[ecwootten]

Configuring is semi-possible (5.801) - it's just that some actions (e.g. mark unread) are always present on the toolbar, and some never appear even if you select them (e.g. move). Although this ticket is marked as 'enhancement', it is masking actual bugs (which have been closed and referred here.

[gree7]

Not sure, whether it should be part of this issue, or another one, but a possibility to create custom action buttons (e.g., Move to a selected folder) could help some workflows.

[lohgun]

Reading this and the linked issues it seems like that I'm the only one that doesn't use the "Mark unread" button at all. However here I am, asking to make this button the same as the other ones that can be disabled in "Visible message actions".