add(roadmap): planned built-in Element implementations

Roadmap.md

@@ -46,8 +46,6 @@ The following list contains goals for certain features the library supports or i
       - [x] horizontal center (i.e. calculate the corresponding anchor)
       - [x] vertical center (i.e. calculate the corresponding anchor)
     - [x] User input
-      - [x] single line
-      - [x] multi line
     - [ ] image support through kitty protocol (**later**)
     - [ ] embeeding another application (i.e. an editor, or something like that)? (**later**)
 - [ ] Examples
@@ -58,11 +56,22 @@ The following list contains goals for certain features the library supports or i
     - [x] mixed (some sort of form)
   - [ ] Elements
     - [x] `Container` Alignment
-    - [x] Button
-    - [x] Text Input field
+    - [ ] Button
+      - [x] custom (see example)
+      - [ ] built-in
+    - [ ] Text Input field
+      - [x] custom (see example)
+      - [ ] built-in single line
+      - [ ] built-in multi line (**later**)
       - [ ] overflow handling, i.e. where to draw the elipse `..` - left, center or right (**later**)
-    - [ ] Popup-menu
+    - [ ] Slider
+    - [ ] Radio Button
+    - [ ] Drop-down menu
+    - [ ] Popup
     - [ ] Stackable / Floating `Container`s / `Element`s
+    - [ ] Form
+      - [ ] basic
+      - support all other built-in `Element`s that display values accordingly
     - [x] Scrollable Content (i.e. show long text of an except of something and other smaller `Container`)
       - [x] min size
       - [x] mouse scrolling aware of mouse position (i.e. through multiple different scrollable `Container`)