Add replace method for all sections

[saschagordner]

This PR adds a new method Form.replaceSubrangeInAllSections(_:with:) which replaces a subrange of all sections (including hidden ones) with a given set of sections. In addition, it replaces existing visible sections within the subrange by the visible sections in the new element collection.

@mats-claassen Please let me know if you have any concerns.

Resolves #2003

[mats-claassen]

The changes look good. I think it would be good to add one or two unit tests for this function. For example something like this:

func testReplaceAllSection() {
        let form = Form() +++ Section("section1") {
            $0.hidden = true
        }
            +++ Section("section2")
            +++ Section("section3")

        form.replaceSubrangeInAllSections(Range<Int>(uncheckedBounds: (lower: 0, upper: 2)), with: [Section("section0") { $0.hidden = true }])

        XCTAssertEqual(form.allSections.count, 2)
        XCTAssertEqual(form.count, 1)
        XCTAssertEqual(form[0].header?.title, "section3")
        XCTAssertEqual(form.allSections[0].header?.title, "section0")
        XCTAssertEqual(form.allSections[1].header?.title, "section3")
    }