[0.14.0] Add explanatory text and a pause before allowing deletion of all sources

[zenmonkeykstop]

@deeplow raised a yellow flag re: Select All and potential data loss, see: #2282 (comment)

tl;dr: is that it is very easy to select all sources and delete them, and so adding in even minimal guardrails should be a release prerequisite. The current plan is to:

• Update the delete sources dialog text when all sources are selected, to clearly communicate that to the user #2298
• Add a timer to delay activation of the delete dialog's "YES,..." button when multiple sources are selected  #2297