Improved Directory Dropdowns.

- Each `DirectoryDropdown` now remembers the longest/most-recent path (navigating with keys).
- Separated mouse and keyboard directory dropdown selection.
- Clicking, scrolling and tapping no longer dismiss directory dropdowns.
- Added a small border on the left on the keyboard-highlighted dropdown selection.
- Fixed bug with dropdown scrolling with Model Info View.
- Encapsulated model directories formally in new `ModelDirectories` data structure.
- Moved download button in the Download Tab.

web/model-manager.css

@@ -222,6 +222,7 @@
     overflow-wrap: break-word;
     overflow-y: auto;
     padding: 20px;
+    position: relative;
 }
 
 .model-manager .model-info-container {
@@ -450,13 +451,13 @@
 }
 
 .model-manager .search-dropdown {
-    position: absolute;
     background-color: var(--bg-color);
     border: 2px var(--border-color) solid;
+    border-radius: 10px;
     color: var(--fg-color);
     max-height: 30vh;
     overflow: auto;
-    border-radius: 10px;
+    position: absolute;
     z-index: 1;
 }
 
@@ -477,10 +478,15 @@
     display: none;  /* Safari and Chrome */
 }
 
-.model-manager .search-dropdown > p.search-dropdown-selected {
+.model-manager .search-dropdown > p.search-dropdown-key-selected,
+.model-manager .search-dropdown > p.search-dropdown-mouse-selected {
     background-color: var(--border-color);
 }
 
+.model-manager .search-dropdown > p.search-dropdown-key-selected {
+    border-left: 1mm solid var(--input-text);
+}
+
 /* model manager settings */
 .model-manager .model-manager-settings > div,
 .model-manager .model-manager-settings > label {