Fix several CSS compatibility issues in Safari (#841)

* Fix button margin default for Safari compatibility

* Add Safari vendor prefixes that are somehow still necessary

* Add workaround for Safari not rendering text selection

* Replace <h3> and <p> placeholder labels in floating menus with <TextLabel>

* Replace <span> elements with <TextLabel> and set its cursor for Safari

frontend/src/components/widgets/inputs/TextInput.vue

@@ -7,7 +7,7 @@
 		:spellcheck="true"
 		:disabled="disabled"
 		:tooltip="tooltip"
-		:style="minWidth > 0 ? `min-width: ${minWidth}px` : ''"
+		:style="{ 'min-width': minWidth > 0 ? `${minWidth}px` : undefined }"
 		:sharpRightCorners="sharpRightCorners"
 		@textFocused="() => onTextFocused()"
 		@textChanged="() => onTextChanged()"