Better (or context-aware) delete/move objects in Layers and Objects dialog

Summary:

Currently, the buttons ('delete', 'move up' and 'move down') in the layer dialog act on the selected object. This produces unexpected results in some edge-cases (see Steps to reproduce) - and requires layers to be 'selected' in order to delete/move them (inkscape#2734 (closed)). The latter issue is handled by 'selecting' layers in the first click (inkscape@9ffd0fb0), which seems to cause inkscape#3527 (closed) and inkscape#3839 (closed).

Proposal

• revert layer 'selection' to second click
• make buttons act differently depending on mode and selection

Mode	Selection	Action	Result
Show-Objects	Yes	app.delete-selection	Delete Selected Object
Show-Objects	No	win.layer-delete	Delete Current Layer
Layers-only	Yes	win.layer-delete	Delete Current Layer
Layers-only	No	win.layer-delete	Delete Current Layer

(moving also would work similarly). While this won't fix it, with layer 'selection' taking two clicks, inkscape#3527 (closed) will not happen as frequently as it does (and would imo be the expected behavior when it does). This would also handle the edge-cases (but, judging from the number of reports, these are rare and low-priority bugs)

Steps to reproduce:

Layers-Only Mode:

• Create two layers L1 and L2
• Create a few objects in L2
• Reorder L2 using Layers dialog
• Nothing happened / selection was moved forward/backward
• Deselect all objects
• Delete layer with using dialog
• Nothing happened / layer has to re-selected for deletion

Objects Mode:

• Make two layers
• Create objects in L2
• Deselect everything
• Delete/Move in Layers dialog
• Nothing happened (But current layer is highlighted and therefore expected to be moved/deleted)

Version info

Inkscape 1.3-dev (def5eff1, 2022-10-25)

    GLib version:     2.74.0
    GTK version:      3.24.34
    glibmm version:   2.66.5
    gtkmm version:    3.24.7
    libxml2 version:  2.10.3
    libxslt version:  1.1.37
    Cairo version:    1.17.6
    Pango version:    1.50.11
    HarfBuzz version: 5.3.1

    OS version:       Windows 10 21H1