Buttons `select area to analyse` must throw warnings `please select image in 'tbl_dir_files'` when no row is selected

[Claudius-Appel]

If no row is selected, these buttons will just do nothing. Instead, they must issue shinyWarnings.

mockup:

if (is.na(input$row_selected)) {
shinyNotification()
}
req(input$row_selected) # the rest of the callback requires this parameter to be set.

[Claudius-Appel]

Additionally, the buttons are now simply not available until tbl_dir_files is populated.