vimiv: Add batch marking images

Use `V` in the library or thumbnail view to mark contiguous images,
mimicking vim visual selection mode.

.gitmodules

@@ -4,3 +4,6 @@
 [submodule "scripts/.local/share/uoeia"]
 	path = scripts/.local/share/uoeia
 	url = https://git.martyoeh.me/Marty/uoeia.git
+[submodule "multimedia/.local/share/vimiv/plugins/batchmark"]
+	path = multimedia/.local/share/vimiv/plugins/batchmark
+	url = https://github.com/jcjgraf/BatchMark

multimedia/.config/vimiv/keys.conf

@@ -31,6 +31,7 @@ b : set statusbar.show!
 tl : toggle library
 tm : toggle manipulate
 tt : toggle thumbnail
+V : batchmark-toggle
 
 [IMAGE]
 M : center
@@ -85,6 +86,7 @@ H : set library.width -0.05
 $ : end-of-line
 <button-right> : enter library
 ^ : first-of-line
+0 : first-of-line
 <ctrl>d : scroll half-page-down
 <ctrl>u : scroll half-page-up
 <button-back> : scroll left

multimedia/.config/vimiv/vimiv.conf

@@ -43,7 +43,7 @@ left_thumbnail = {thumbnail-index}/{thumbnail-total} {thumbnail-basename}{read-o
 left_manipulate = {basename}   {image-size}   Modified: {modified}   {processing}
 center_thumbnail = {thumbnail-size}
 center = {slideshow-indicator} {slideshow-delay} {transformation-info}
-right = {keys}  {mark-count}  {mode}
+right = {keys} {batchmark} {mark-count}  {mode}
 right_image = {keys}  {mark-indicator} {mark-count}  {mode}
 
 [KEYHINT]
@@ -71,6 +71,7 @@ shuffle = False
 [PLUGINS]
 print = default
 metadata = default
+batchmark =
 
 [ALIASES]