String: Add .is-empty and .character-count properties

Introduce two new properties for string in .slint:
- .is-empty: Checks if a string is empty.
- .character-count: Retrieves the number of grapheme clusters
  https://www.unicode.org/reports/tr29/#Grapheme_Cluster_Boundaries

These additions enhance functionality and improve convenience when working with string properties.

api/cpp/Cargo.toml

@@ -60,6 +60,7 @@ raw-window-handle = { version = "0.6", optional = true }
 
 esp-backtrace = { version = "0.14.0", features = ["panic-handler", "println"], optional = true }
 esp-println = { version = "0.12.0", default-features = false, features = ["uart"], optional = true }
+unicode-segmentation = "1.12.0"
 
 [build-dependencies]
 anyhow = "1.0"

api/cpp/lib.rs

@@ -152,6 +152,11 @@ pub extern "C" fn slint_string_to_float(string: &SharedString, value: &mut f32)
     }
 }
 
+#[no_mangle]
+pub extern "C" fn slint_string_character_count(string: &SharedString) -> usize {
+    unicode_segmentation::UnicodeSegmentation::graphemes(string.as_str(), true).count()
+}
+
 #[no_mangle]
 pub extern "C" fn slint_string_to_usize(string: &SharedString, value: &mut usize) -> bool {
     match string.as_str().parse::<usize>() {